MongoDB uses a range based sharding approach. If you choose to use hashed based sharding, you must have a hashed index on the shard key and cannot drop it since it will be used to determine shard to use for any subsequent queries ( note that there is an open ticket to allow you to drop the _id index once hashed indexes are allowed to be unique SERVER-8031 ).
As to why the query appears to be using the _id index rather than the _id_hashed index - I ran some tests and I think the optimizer is choosing the _id index because it is unique and results in a more efficient plan. You can see similar behavior if you shard on another key that has a pre-existing unique index.